In Synchronous Optical Network/Synchronous Digital Hierarchy (SONET/SDH) Unidirectional One-plus-One APS, the same traffic is transmitted on two ports that are part of the same protection group. User traffic in a transmit direction of a SONET/SDH port therefore runs independently of any defects in the receive direction of the SONET/SDH port, and vice versa. This means that a defect in the receive direction of a SONET/SDH port, for example, cannot disrupt the user traffic in the transmit direction of that port.
In Time Division Multiplexing (TDM) technology, however, when a defect is detected in the receive direction of a T1/E1 circuit, a Remote Defect Indication (RDI) signal is transmitted in the transmit direction of the T1/E1 circuit. A remote end device that receives the T1/E1 RDI signal is required, according to T1/E1 standards, to overwrite the user traffic on the T1/E1 circuit with a fixed Trunk Conditioning (TC) bit pattern. A defect in the receive direction of a T1/E1 circuit, which may be carried by a SONET/SDH port, would therefore lead to disruption of user traffic in the transmit direction of the T1/E1 circuit.
As a result of this incompatibility between SONET/SDH Unidirectional One-plus-One APS requirements and T1/E1 standards, SONET/SDH Unidirectional One-plus-One APS cannot operate properly on a SONET/SDH port that carries and terminates T1/E1 circuits.
Thus, there remains a need for techniques to coordinate APS and error signal processing.